It is often necessary to measure the rate of flow of fluid in a borehole. The present apparatus is an ultrasonic Doppler measuring device providing a fluid velocity measurement of fluid in a borehole. Consider a typical example in which a cased well has been produced for some period of time. Assume further that it may span two or three different horizons having perforations and potentially producing fluids into the borehole. It is necessary to measure the volume of the fluid flowing from each set of perforations at the respective horizons. This can be typically compounded even further when the formations produce different fluids. Assume that one produces only petroleum products while another produces oil and water in a known ratio. Alternately, assume that one of the formations produces natural gas in a specified ratio. In such an example as this, the commingled fluids will flow upwardly in the cased borehole. Routinely, entrained bubbles are carried along with the fluid flow. The bubbles and fluid flow at rates which may be the same or different depending on a number of factors discussed below.
It is often desirable to measure the fluid flow velocity. One device used for this is a type of flow meter which has a propeller affixed to some kind of counting device. The velocity of the propeller driven by the fluid flowing past the measuring device defines the flow rate by measuring the velocity as the propeller is spun. This works in an acceptable range of minimum and maximum flow velocities, but it does not work well at every velocity. There are a number of factors which can obscure measurements and create difficulties in making the measurements.
The present apparatus is a system that can be used to measure fluid velocity in a variety of mixtures and circumstances. It is a system which especially responds to scattered droplets of gas entrained in the form of bubbles in the flowing fluid. Alternately, there are typically entrained particles such as mill scale or other solid particles such as sand from the formation. Whatever the source, there is a strong possibility that the fluid will be something other than a pure fluid. Consider as one example a well which produces natural gas commingled with other petroleum products. The entrained gas bubbles are carried along with the produced fluid and they may indeed even flow faster than the produced liquids. The bubbles add another form of interface which scatters ultrasonic energy, a feature exploited as described below.
The disclosure sets forth a pulsed ultrasonic Doppler system which takes advantage of scattering from droplets and other particles in the flowing fluid stream. It is a system ideally located at the lower end of a centralized instrument body located in a cased well borehole, and wherein the casing defines the fluid path for the mixture of fluids flowing up the well. An instrument package of specified diameter is centralized by upper and lower sets of centralizers so that the fluid produced by the formation(s) flows up the casing and around the body which houses or holds the instrument described below. In this circumstance, it is possible to obtain the flow rate by directing an ultrasonic pulse from the measuring instrument downwardly into the fluid flow directed at the volume of fluid below the tool.
Pulses are formed at a selected repetition rate and have a short pulse duration; they are transmitted downwardly into the flowing fluid and impinge on reflective surfaces. Scattering occurs either by reflection or refraction. This involves the interface between various matrials making up the flow. For instance, droplets of oil and water will provide such an interface. Gas bubbles in an otherwise liquid flow will also provide an interface. Sand, mill scale and other particles of a solid nature also provide such an interface. The scattering has the form of a reflected signal after the transmitted pulse. It is therefore received at a time interval thereafter, and encodes the movement of the scattering particles in the fluid in the form of a Doppler shift. The Doppler shift can be measured and calibrated to obtain fluid flow velocity.
Several velocities may be involved in the relative measurement. For instance, the measuring tool can either be fixed or moving. The fluid can either be stagnant or moving. One ordinary circumstance will find the measuring tool moving downwardly while the fluid is flowing upwardly. The rate of movement of the measuring tool is normally obtained at the surface where it is measured as the logging cable for the measuring tool is lowered into the cased well, and that measurement can be readily subtracted from measurements of the fluid velocity relative to the tool. Alternately, the velocity of the tool relative to the stationary surrounding casing can be measured. In any event, such measurements can be made.